The present invention generally relates to a system for correcting an error through decoding of an error correction code and, more particularly, to an error correction decoder which performs its error correcting operation on a pipeline processing basis for each code block and which is suitably used in a digital magnetic recording/playback apparatus.
In the case of such a recording medium as magnetic tape on which a digital signal is recorded, when it is desired to play back the recorded data, e.g., "1" and "0", scars, scratches or dust on the tape sometimes cause erroneous playback of them as "0" and "1". As its countermeasure, there has been known a system in which data is divided into a plurality of blocks, each having a predetermined length, and parity data is added to each block to form a codeword for an error correction code and the data is recorded in the form of such codewords. In this case, its playback system includes an error correction decoder which sequentially performs its error correcting operation over each of the codewords read to realize correct playback of the data.
The aforementioned error correcting operation includes a plurality of processing steps of generating a syndrome, deriving an error evaluation polynomial and an error location polynomial and evaluating error locations and error values. In a typical conventional error correction decoder, the decoder has a plurality of blocks which execute the plurality of processing steps respectively as an allocated portion thereof, so that the plurality of blocks perform their pipeline processing operations over a plurality of codewords that are continuously received. Each block, according to a common control signal, transfers its processing result to the next block and starts processing of the next codeword. Such an example is an error correcting system as disclosed in JP-A-60-223334.
FIG. 2 shows a block diagram of a prior art error correcting system disclosed in The Journal of The Institute of Television Engineers of Japan Vol. 43, No. 12 (1989), pp.1333-1339. In the error correcting system, among the aforementioned plurality of processing steps of the error correcting operation, the processing steps of deriving error evaluation/location polynomials and evaluating error locations and values are allocated to an error location/evaluation block 12. More specifically, the error correcting system comprises a syndrome generation block 11, the error location/evaluation block 12 and a correction block 13. The codeword processings of the respective blocks are started under the control of a common reset signal indicative of the leading head of the codeword. Transfer of the codewords between the blocks in the error correcting system is illustrated in FIG. 3. The reset signal is generated by detecting a synchronizing pattern recorded in the leading head of the codewords on a recording medium.
In such a system of correcting an error in each codeword based on the detected synchronizing pattern as mentioned above, generation of an out-of-synchronism condition causes generation of an incomplete codeword that is shorter than the normal codeword. With a helical scan type digital video tape recorder (VTR), in particular, reading operation is undesirably effected across a plurality of tracks having a codeword string recorded therein in a special playback mode, e.g., in a high-speed playback mode. Such a track shift causes an out-of-synchronism condition, thus resulting in generation of an incomplete codeword that is shorter than the normal codeword. At this time, pulse interval in the reset signal is correspondingly shortened. With the above error correcting system, when the reset interval is shortened in this way, the processings of the error location/evaluation block 12 and correction block 13 are also aborted at the same interval. The codeword then being processed at the error/location/evaluation block is subjected at the correction block 13 to a wrong correcting operation based on intermediate processing values. Further, the codeword issued from the correction block 13 is missing data because the correction block 13 shifts its correcting operation toward the next codeword in the course of its output operation. Such conversion of the codeword data to wrong data based on the wrong correction or data missing is referred to as "mis-correction". Such mis-correction becomes a serious problem, for example, in picture image data because the mis-correction results in deterioration of its picture quality.